The invention relates to the determination of the rotational movement of an object and especially to a preferable way of providing duplication of speed measurement related to machine safety.
Conventional non-contacting speed measurements are based on inductive, capacitive, magnetic, or light-employing solutions in which a detector, in other words a short-distance proximity switch, is typically set very close to the axle of a rotating object, and the passing of a counterpart or counterparts fastened to the cylindrical surface of the axle can be accurately detected as detector pulses. This counterpart can be characterized as a passive part of the system. In this type of speed measurement, a typical pulse frequency is one pulse per axle rotation, in which case the measurement is called a sparse pulse measurement. Due to the nature of the sparse pulse measurement, it is not very accurate, and there are delays in the measurement when the speed changes.
The purpose of such a measurement of rotational movement is to provide information on the actual rotational movement and not so much an accurate measuring result of the exact speed of the rotation that could be used as feedback information for speed control, for instance. The task of the conventional sparse pulse measurement is typically to produce a second measuring result related to machine safety. In certain operating conditions, two independent measurements of the rotational speed are often required, and these relate to the operation of safety equipment, for instance.
For the duplication of the measurement, it is unnecessary to implement both duplicate measurements with exact and often high-cost detectors, when the second measurement is only used to detect the rotational movement.
Contacting tachometers and pulse detectors are generally used for performing accurate measurements. In addition, a magnetized or other non-contacting code ring can be fixed to the axle. The use of such a device corresponds to said contacting solutions in view of its relatively high costs and difficulty of installation.
To achieve machine safety and duplicate measurements, it is thus necessary to produce a reliable measuring method that is simple to install and inexpensive and reliable.